Band gap determination in lead iodide polytypes

The band gaps of the polytypic crystals of lead iodide have been determined by studying the temperature variation of conductivity. Results obtained on several polytypic crystals show that the band gaps of the higher polytypes are lower than that of the parent 2H structure (2.56 eV). As for example the band gaps for 12H, 14H, 18H and 20H (disordered) were found to be 1.65, 1.44, 1.07 and 1.01 eV. The significance of the band gap determination in regard to the recent anomalous results on the physical properties of lead iodide crystals, has been indicated.